The present invention relates to a method and apparatus for in situ determination of permeability and other formation characteristics and more particularly to such a method and apparatus employing expandable packers for defining a test interval in a borehole.
In situ measurements of permeability in various underground formations have long been of importance. For example, such studies have been conducted in oil wells and the like as well as in mining operations, particularly those using solution mining or leaching operations. Solution mining has been employed for example in recovery of metals such as copper, uranium, lead, zinc and nickel. In connection with such mining operations, it is essential to accurately characterize the permeability of the underground formations of interest in order to determine the degree of effectiveness possible for techniques such as solution mining or leaching operations.
In addition, in situ permeability studies are of substantial importance in connection with the development of underground storage facilities for waste nuclear materials. Commonly, storage facilities are developed in extensive underground formations characterized by minimum permeability in order to assure that the materials will not leach or seep into the underground formation and escape from the immediate area of the storage facility over long periods of time.
In the prior art, substantial effort has been expended in developing techniques for characterizing permeability of such underground formations by the study of fluid flow characteristics within the underground formation. The relationship between permeability and flow characteristics has been well established, for example, under Darcy's Law or modifications thereof which define the relationship of permeability in connection with fluid flow, either liquids or gases, through a substrate under study in response to a given pressure differential or head. Additional parameters such as porosity, saturation, fluid viscosity, threshold pressures, temperature, previous testing history, fracture extent, etc. may be of importance in accurately determining such permeability values.
In essence, Darcy's Law and the like provide a means for calculating permeability in darcy units as well as other formation characteristics depending upon fluid flow characteristics measured in the substrate under question. Calculations of the type referred to above are well known in the prior art and accordingly are not set forth in greater detail herein. If desired, greater detail concerning such calculations and permeability studies in general are set forth for example in a report prepared for the Bureau of Mines, Department of the Interior, Washington, D.C., entitled, "Field Permeability Test Methods With Applications to Solution Mining," Report No. BuMines OFR 136-77, published August 1977 and further identified by Accession No. PB 272 452. That report includes a survey of existing and developing field permeability test methods conducted in order to identify methods suitable for use in feasibility investigations or performance monitoring in connection with in situ leaching of ore deposits. However, the permeability test methods disclosed in that report are equally adaptable to other applications such as those described above.
Permeability tests are commonly conducted in boreholes extending into the underground region of concern. The borehole may extend downwardly from the surface or even outwardly in any direction from underground tunnels or shafts. Flow characteristics providing means for calculation of permeability, porosity, etc. are determined by maintaining pressure within the borehole at a differential either above or below the ambient pressure of the surrounding formation. With the pressure in the borehole being below that of the surrounding formation, fluids from the surrounding formation tend to flow into the borehole. Such techniques are commonly referred to as "in-flow" tests. In such tests, the fluid flow may be either in the form of gases and/or liquids. Similarly, the borehole may also be pressurized above the ambient pressure for the surrounding formation so that fluids from the borehole tend to flow or "permeate" into the formation. Techniques of this type are commonly referred to as "outflow" tests. In tests of either the inflow or outflow type, the rate of flow of fluids into or out of the underground formation provides the basis for calculating permeability of the formation. Inflow and outflow tests of the type referred to above are also widely known in the prior art, examples being set forth for example in the Bureau of Mines report referred to above.
In conducting such tests, it is necessary to isolate a selected test interval of predetermined length at a selected location within the borehole. Such tests may be conducted at various levels in the borehole in order to completely characterize the underground formation at various depths beneath the surface. In any event, it has also become common in the prior art to employ various types of expandable packers for forming seals at various points along the borehole in order to define such isolated regions or test intervals. These packers may be of either an inflatable or a mechanical type, the inflatable packer being inflated by liquid, air or other gases in order to expand the packer into sealed engagement with the borehole. Similarly, mechanical packers are also known which are mechanically expanded through various mechanisms for similarly urging an annular seal surface of the packer into engagement with the borehole. With a pair of packers being arranged in predetermined spaced-apart relation at a given location within the borehole, an isolated region or test interval is then defined between the packers wherein permeability studies or tests of the type referred to above may be conducted. As noted above, the test interval formed between the packers may be either placed under pressure greater than the ambient pressure in the surrounding formation or evacuated to a pressure below that of the surrounding formation in order to produce either outflow or inflow test conditions as were also summarized above.
A variation of such testing procedures is commonly referred to as "whole hole testing" where a test interval is formed between a single expandable packer and the end of the shaft. Tests of this type may be used, for example, to determine formation characteristics at different locations as the borehole is being drilled or formed.
The particular construction of the packers themselves is not a feature of the present invention. Typical packer constructions may be seen for example in U.S. Pat. No. 3,876,003 issued to Kisling III on Apr. 8, 1975, U.S. Pat. No. 3,565,172 issued to Cole on Feb. 23, 1971 and U.S. Pat. No. 3,439,740 issued to Conover on Apr. 22, 1969. Each of these patents, particularly the first and last patents noted above, discloses packer assemblies of the type contemplated by the present invention.
Various techniques for carrying out flow studies resulting in the determination of permeability values for the surrounding formations are described at length in the prior art, for example, within the Bureau of Mines report referred to above. The use of such packers and the conducting of flow tests within isolated regions or test intervals provides an effective indication of permeability values for the surrounding formation. Past efforts in connection with permeability studies have tended to result only in an overall permeability value for the surrounding formation. However, underground formations are characterized by multi-directional components of permeability which may have a substantial effect on various applications contemplated for the underground formation. For example, flow between the test interval defined or isolated by the packers and the surrounding formation depends in large part upon formation permeability in radial orientation relative to the borehole. The prior art has recognized this to the extent that the test interval formed between the packers has often been extended in order to diminish "end effects" resulting from axial flow, that is, flow parallel to the length of the borehole, at opposite ends of the test interval. Such axial flow may result from permeability within the formation itself as well as from leakage around the packers due to improper sealing of the packer against the borehole walls or from axial striations extending along the borehole walls adjacent the packers. Any of these characteristics may provide an axial flow path permitting some axial fluid flow between the formation and the test interval. Furthermore, such axial fluid flow may bypass the packers so that part of the observed flow passes either from or into the borehole outside the defined test interval. Generally, such conditions, usually termed "end effects," tend to interfere with accurate characterization of permeability for the formation.
Some effort has been made in the prior art to overcome this problem and to characterize formation permeability while cancelling the effect of such axial flow, particularly that caused by packer leakage or striations in the borehole walls. For example, one such effort involved the use of two additional packers arranged in respective spaced-apart relation with the packers forming the test interval. The additional packers thus defined additional cavities at opposite ends of the test interval. According to the prior art, these two additional cavities are then placed in communication with each other, water being injected into the two additional cavities in order to maintain them at the same pressure as that observed within the test interval formed between the two primary packers. The purpose of this four-packer arrangement, with the two additional cavities, was to assure flow of fluid from the central cavity or test interval outwardly in generally radial flow into the surrounding formation. In other words, the above four-packer system was proposed and tested in order to cause flow from the test interval to be controlled by pressurization of the two additional cavities in order to satisfy an assumption of flow only in the radial direction from the central test interval. Similarly, yet another modification contemplated in the prior art was the use of three packers forming two adjacent test intervals or cavities, water being injected into one of the test cavities to provide a source, water being pumped from the other cavity in order to provide a "sink." This technique is not discussed in greater detail herein since it is believed obvious that the following discussion of shortcomings for the above four-packer system also applies to this three-packer system.
Referring again to the four-packer system described above, it was possible to further characterize lateral or radial permeability of the surrounding formation. However, tests of the type contemplated in connection with the four-packer system and in connection with all of the prior art techniques referred to above generally concerned flow rates and permeabilities of a relatively high level. For example, two, three or four-packer systems of the type provided by the prior art are generally very effective in determining permeabilities in the range above 1,000 microdarcies and, at least in some applications, even down toward a level of approximately 100 microdarcies. Measurements of this magnitude are very satisfactory for many applications such as those commonly encountered in permeability studies in connection with oil and gas field technology, in classical hydrology and the like.
However, it is becoming of greater importance to conduct permeability studies in underground formations which are considered as classically impermeable media, or which are of low permeability such as salt formations, shales, hydrites, limestone formations and the like. For example, such underground formations are commonly encountered in the formation of underground storage for nuclear waste materials and in some current applications for solution mining.
In such applications, it is commonly necessary to identify permeability values in ranges well below 1,000 microdarcies and even well below 100 microdarcies. More specifically, it may be necessary to identify permeability values in the range of 10 microdarcies and even substantially lower.
It will be immediately apparent that in a conventional borehole configuration, the determination or inference of such low level permeability values necessarily involves measurement of fluid flow at similarly reduced rates. With fluids flowing into and out of the test interval at these greatly reduced rates, any axial flow components produced either by a misfit of the packer, by striations in the borehole wall or even by axial permeability within the surrounding formation have a much greater tendency to affect and prevent accurate interpolation of radial permeability values.
Even the four-packer test system referred to above may be inadequate for accurately measuring flow characteristics necessary for precisely determining permeability values in such a situation. For example, within the four-packer system, it may be assumed that both of the packers arranged below the central test interval may have some leakage characteristics. Within the four-packer test procedure referred to above, pressurization of the additional cavity between the two leaking packers would tend to prevent detection of the leakage. At the same time, some fluid from the central test interval could flow axially past the two leaking packers to interfere with accurate characterization of flow and permeability values for the formation surrounding the central test interval.
In any event, prior art techniques such as the four-packer system referred to above have attempted to overcome the effects of multi-directional permeability by counteracting or balancing all but one directional vector. Note the reference above to enforced radial flow. However, in many applications, particularly those requiring characterization of permeability at very low darcy or microdarcy values, such treatment is not satisfactory and provision must be made for accurately and precisely characterizing the different directional components of permeability for the formation.
Thus, prior art techniques have attempted to resolve the problem of axial permeability flow or leakage flow by making measurements with a system preventing axial flow of the fluid under analysis, that is, the fluid flowing into or out of the central test interval. However, in view of the preceding remarks, it is immediately apparent that more precise knowledge of such axial permeability or flow components may be necessary in order to precisely characterize permeability values for the formation. Accordingly, there has been found to remain a need for a more precise method and apparatus for characterizing permeability in underground formations and more particularly in such formations surrounding boreholes.